Facebook said it was working with the US Federal Bureau of Investigation (FBI) and the Metropolitan Police Service.

"While no user data was [compromised], we have been working with Scotland Yard and the FBI, as we take any attempt to hack our internal systems extremely seriously," Facebook spokeswoman Sophy Tobias said in an e-mail message. "However, we have no further comment as this is an ongoing criminal investigation."

A spokesman for the Metropolitan Police said that the 26-year-old man had been arrested in North Yorkshire last Thursday on "suspicion of computer hacking offenses," and was now released on bond. The spokesman declined to provide further details on the case. The FBI could not be reached for comment.

It's not clear what the motive for the attack may have been, but Facebook is an attractive target for hackers. Because the site connects people who know and trust each other, it is often targeted by scammers and hackers looking to trick users into clicking on links or installing malicious software.

The site also contains a wealth of personal information that could be used in specially targeted spear-phishing attacks. Spear-phishing attacks involve specially crafted e-mail messages that are written to look like they come from someone the victim knows.